Although cell components of blood include an erythrocyte, a leukocyte and a blood platelet, on rare occasion, a cell other than these cells may exist in the blood. An example thereof is a circulating tumor cell (CTC). It is considered that metastasis of cancer occurs since a cancer cell is conveyed through blood vessels or lymph vessels to any other site in the body and proliferates there. It has been reported that the number of CTCs (circulating tumor cells) in the blood correlate to the possibility of metastasis and prognosis, and thus it has been known that the number of CTCs in blood and a nucleic acid of a CTC are measured for providing a guideline for prediction or decision in diagnosis, prognostication, and a therapeutic effect of cancer (in particular, metastatic cancers such as breast cancer) (see Circulating Tumor Cells; Evolving Evidence and Future Challenges, The Oncologist 2009; 14; 1070-1082).
Examples of techniques for separating and detecting CTCs in blood include: a method of capturing and separating CTCs in blood by use of an antibody with respect to a CTC specific surface antigen (see JP 3834326 and JP 2007-178193 A); a separation method using adhesion (see WO 2005/043121 and WO 2006/078994); a separation method using a density gradient (see JP 2010-075073 A and WO 95/20429); a separation method using a filter (see WO 2006/116327 and WO 2008/155398); a method of measuring the telomerase activity of a CTC (see WO 2010/071114); a separation method through hemolysis by using a hypotonic solution (see WO 2004/056978); and a method utilizing flow cytometry (see WO 2008/057437). For detecting, quantifying and counting the object separated, measurement of a nucleic acid is generally carried out by utilizing PCR or any substance having affinity to a CTC; optical CTC measurements are carried out as a combination of an antigen and/or a fluorescent pigment.